1. Field of the Invention
The present invention relates to measurement of deployed cable lengths, and, more particularly, to measuring wheels for indicating the depth of a wellbore logging tool deployed on an armored cable known as a wireline.
2. Background of the Related Art
Hydrocarbon exploration and drilling operations commonly employ logging tools for interrogating subsurface formations intersected by a wellbore. As shown in FIG. 1, a logging tool 2 is positioned in a wellbore 4 using an armored cable known in the art as a wireline 6. Typically, a wireline is connected at one end to a logging tool 2 and connected at its other end to a spooling winch 8 positioned at the surface of the land or body of water overlying the subsurface formation F of interest. The spooling winch 8 is used to control the movement of the wireline 6, and thus the logging tool 2, into and of the wellbore 4. The data obtained by a logging tool is specific to the depth at which the logging tool is positioned when the data is collected. It is therefore essential to continuously determine the depth of the logging tool while the tool is collecting wellbore data.
It is well know in the relevant art to determine the depth of a logging tool 2 by measuring the length of the wireline 6 supporting the tool, particularly the length of wireline extending from the deployment surface (land or water) to the logging tool. Measurement of this length is typically accomplished with a mechanical wheel system, sometimes in combination with a magnetic marking system. The mechanical wheel system employs a measuring wheel 9 having a calibrated outer radial surface, or perimeter, over which the wireline is at least partially wrapped. The frictional engagement between the wireline and the measuring wheel's perimeter results in rotation of the measuring wheel as the wireline is moved into and out of the wellbore by the spooling winch. Revolutions of the measuring wheel are counted and correlated to depth using the circumference of the measuring wheel, and recorded and/or displayed on a visual indicator.
Measuring wheels of this sort are currently made of stainless steel, Invar or some other metal that are chosen for their resistance to physical change during operations, because the physical dimensions of the wheel—particularly the radius—directly affect the determined wireline depth. Thus, characteristics such as low coefficient of thermal expansion and high toughness are desirable. Low magnetism (i.e., nonmagnetic) characteristics are further desirable to avoid interference with logging tools that sense magnetic properties of the subsurface formations.
Known measuring wheels must be frequently replaced or refurbished due to wear caused by movement and nibbing of the wireline over the measuring wheel before and after frictional engagement between the two. The calibration between the measuring wheel's outer radius and its indicated depth is very sensitive to wear: even a reduction of 0.001 inches in the radius of the measuring wheel will significantly affect the accuracy of the resulting depth indication. Since depth determination of a well logging tool is critical in identifying the location of subsurface hydrocarbon formations, wear by the measuring wheel can contribute to a laterally steered wellbore missing a desirable formation.
Wear of a measuring wheel's perimeter also permits slippage between a wireline and the measuring wheel, further adversely affecting depth determination.
U.S. Pat. Nos. 5,351,531; 4,718,168; and 4,117,600 disclose typical uses of measuring wheels in wireline systems, but are directed to supplemental systems for improving the accuracy of the depth indication provided with measuring wheels, and fail to address the above-noted wear issue.
It is therefore desirable to provide a measuring wheel that is more resistant to wear than known measuring wheels, whereby the reliable service life of the measuring wheel is increased.
It is further desirable to make the perimeter of a measuring wheel more resistant to slippage by a wireline wrapped at least partially over the perimeter.
It is further desirable to provide a method for refurbishing a spent measuring wheel in a manner that improves the measuring wheels wear resistance and slippage resistance, as opposed to discarding the spent measuring wheel in favor of a newly fabricated measuring wheel.